Some crystalline aluminosilicates, or zeolites, are useful as adsorbents in separating a particular hydrocarbon compound from mixtures of hydrocarbons containing the compound. In particular, zeolites are widely used for selective separation of paraxylenes from mixtures containing other C.sub.8 aromatic compounds such as metaxylene, orthoxylene, or ethylbenzene. For example, U.S. Pat. Nos. 3,636,121; 3,686,342; 3,686,343; 3,835,043; 3,855,333; 3,878,127; 3,894,108; 3,903,187 and 4,265,788 are all directed towards a method of removing paraxylene from mixtures or of selectively separating paraxylene and ethylbenzene from mixtures containing other components, using various types of zeolites as adsorbents. Paraxylene is a commercially important aromatic hydrocarbon isomer since its use in the manufacture of terephthalic acid is a critical step in the subsequent production of various fibers such as Dacron.
This invention, however, relates to a process for separating ethylbenzene from a feed mixture containing ethylbenzene and at least one other xylene isomer and is therefore unrelated to paraxylene separation processes. Additionally, in the process disclosed herein, ethylbenzene is selectively adsorbed in relation to the less selectively adsorbed xylene isomers.
While a separation of paraxylene from other xylene isomers is desirable in certain circumstances, it has become increasingly desirable to recover ethylbenzene from streams containing both ethylbenzene and xylene isomers. Ethylbenzene has great commercial importance since it is a building block in the production of styrene. Further, the cost of producing ethylbenzene by the reaction of benzene with ethylene has steadily increased. These costs have prompted research efforts in the recovery of ethylbenzene from various C.sub.8 aromatic feedstreams which already contain ethylbenzene. Such feedstreams may be C.sub.8 aromatic extracts resulting from various solvent extraction processes, from pyrolysis gasoline, or from reformed naphtha.
It is known that zeolite Beta has been used to adsorb mixtures of paraxylene and ethylbenzene selectively from mixtures comprising ethylbenzene, orthoxylene, metaxylene and paraxylene using toluene as a desorbent. See U.S. Pat. No. 3,793,385 to Bond et al., issued Feb. 19, 1974. Bond et al. additionally suggests a large number of cations including Li, K, Cs, Mg, Ca, Sr, Ba, La and Ce may be included in the zeolite. Cs and K are especially preferred.
However, the invention disclosed herein is based on the discovery that certain desorbents modify the behavior of zeolite Beta so that it adsorbs ethylbenzene in substantial preference to paraxylene and the other isomeric xylenes. Generically, these desorbents belong either to the family of monosubstituted benzenes wherein the substituent contains a heteroatom selected from the group consisting of O, S, P, and the halogens (particularly halobenzenes; for instance, iodobenzene) and alkylbenzenes with a linear side chain, or to the family of paradialkylbenzenes (particularly, p-ethyltoluene, p-diethylbenzene and p-methyl n-propylbenzene).
Other zeolite systems are known which selectively adsorb ethylbenzene from mixed C.sub.8 aromatic streams in the presence of diethylbenzene as desorbent. One such process is disclosed in U.S. Pat. No. 3,943,182 to Neuzil et al., issued Mar. 9, 1976. However, the zeolites disclosed therein are either Type X or Type Y. The adsorptive activity of a particular type of zeolite is not easily predictable, if it is predictable at all. Indeed, the direction in which zeolite selectivity is affected by a particular desorbent is even less predictable.